Field of the Invention
The present disclosure is directed to valves for railway cars and, more particularly, to a hot wheel protection valve for a railway car.
Description of Related Art
Current brake systems for railway vehicles experience leakage of pressurized fluid into the brake cylinder of the brake system. The Association of American Railroads' specification for a single railway car specifies allowable leakage into/out-of the brake cylinder at the rate of +/−1-psi/min. At this rate, acceptable loss of brake cylinder pressure is permitted while still safely maintaining brake cylinder pressure levels for the operation of the railway vehicle's brake system. When operating a railway vehicle on a grade for an extended period of time, however, a minimal level of leakage into the brake cylinder in one minute can increase to a high enough pressure to cause excessive heat to be generated by the excessive pressure. This excessive pressure results in higher than desired shoe force creating a “hot wheel” condition in which forces within the brake cylinder that cause the brake cylinder and the wheels of the railway vehicle to experience excessive heat. By venting pressure from the brake cylinder, this “hot wheel” condition is avoided.
An example of a preexisting brake cylinder pressure reducing valve is disclosed in U.S. Pat. No. 5,083,843. The brake cylinder pressure reducing valve is used to control the release of brake cylinder pressure. Upon the graduated release of brake cylinder pressure, brake pipe pressure is restored by a locomotive brake valve. The control valve of the release valve is moved to its full release position and brake cylinder pressure is rapidly reduced. The reduction in brake cylinder pressure reduces a downward force exerted on a diaphragm by the brake cylinder pressure in a first chamber, allowing the graduating volume air pressure to move a stem upward. Thus, the first chamber is sealed and air is retained in the brake cylinder at a value equal to the remaining reduction in brake pipe pressure. Therefore, with the graduating check valve seated, further exhaust of brake cylinder pressure is prevented and the valve remains at a stable position. Brake cylinder pressure can be reduced in a predetermined proportion to a brake pipe pressure increase. Therefore, this release valve is not configured to release brake cylinder pressure upon the brake cylinder exceeding a predetermined amount. The brake cylinder is released as a function of an increase in brake pipe pressure, not through an increase in the brake cylinder pressure.
Another example of a preexisting brake cylinder pressure reducing valve is disclosed in U.S. Pat. No. 6,609,769. A pneumatically graduated brake pressure release valve for a freight train brake system is used to release pressure from a brake cylinder. The graduated release valve controls the exhaust of the brake cylinder pressure. A metering valve portion of the graduated release valve exhausts brake cylinder pressure generally proportional to an increase in brake pipe pressure. On one side, a graduating piston is held in position by pressure from an emergency reservoir. On an opposite side, pressure from the brake pipe and a brake cylinder exhaust urge against the piston. Once the brakes have been applied, if a reduction in brake cylinder pressure is desired, the brake pipe pressure must be increased. This increase in brake pipe pressure unbalances the pressures acting on the piston and causes the brake cylinder to be exhausted to atmosphere. The brake cylinder will only exhaust, however, until the brake cylinder pressure decreases proportionally to the brake pipe pressure increase. The pressure exhausted from the brake cylinder is generally a function of the increase in brake pipe pressure. This graduated release valve is not configured to reduce brake cylinder pressure upon an increase in brake cylinder pressure due to leakage of pressurized fluid into the brake cylinder. Further, this type of graduated release valve does not contemplate the use of a biasing member to counteract the pressures of the brake pipe and the brake cylinder.